Semiconductor strain sensitive devices and method of making the same



Jufiy E3, 397 R. N. HALL SEMICONDUCTOR STRAIN SENSITIVE DEVICES AND METHOD OF MAKING THE SAM Original Filed Dec. 26, 1961 Fig. 3.

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@y M W \SN/XJZL/ His Aimm ey- United States Patent Ofifice 3,331,124 Patented July 18, 1967 3,331,124 SEMICONDUCTQR STRAIN SENSITIVE DEVICES AND METHOD OF MAKING THE SAME Robert N. Hall, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York Original application Dec. 26, 1961, Ser. No. 161,964, now Patent No. 3,292,128, dated Dec. 13, 1966. Divided and this application June 9, 1966, Ser. No. 556,391

6 Claims. (Cl. 29-577) This application is a division of my copending application Ser. No. 161,964, filed Dec. 26, 1961, now Patent No. 3,292,128, and assigned to the assignee of this application.

This invention relates to semiconductor strain sensitive devices and to a method of making such devices having improved characteristics. More particularly this invention is related to a specific embodiment of the complete bridge-type strain sensitive elements, of the general type disclosed and claimed in the copending application of Gunther E. Fenner, Ser. No. 104,271, filed Apr. 20, 1961, now Patent No. 3,251,222 and assigned to the assignee of the present invention, which has improved characteristics. In this respect, therefore, the present invention is an improvement over the invention of the above-identified patent application which invention was made by said Gunther E. Fenner prior to my invention. I, therefore, do not herein claim anything shown or described in the said Fenner application, which is to be regarded as prior art with respect to this present application.

Many problems are presented in suitably applying strain sensitive elements to the strained members for measuring loads. Cements may creep and mechanical linkages may slip or exhibit mechanical hysteresis. Further, in the improved complete bridge-type semiconductor strain sensitive elements of the above referenced Fenner application, the bridge arms should usually be very thin, and the semiconductive material thereof highly impregnated with impurity, in order to obtain a desired electrical impedance and achieve optimum strain sensitive properties. These two latter requirements contribute to the difliculty of obtaining a high impedance, complete bridge unit utilizing heavily impregnated semiconductive material.

It is an object of this invention, therefore, to provide a strain sensitive device which substantially overcomes one or more of the prior art ditficulties and which lends itself to achieving greater sensitivity.

It is another object of this invention to provide a strain sensitive device wherein the semiconductor strain sensitive elements are formed of, and Within, the strained member itself.

It is a further object of this invention to provide a complete bridge-type semiconductor hydrostatic pressure measuring device wherein two arms of the bridge are loaded in tension while the remaining two arms are loaded in compression.

It is a still further object of this invention to provide a complete bridge-type semiconductor strain sensitive device which allows for greater sensitivity and linear range of response.

Briefly stated, in accordance with one aspect of this invention, an improved semiconductor strain sensitive device comprises a monocrystalline body of high resistivity semiconductive material having a preselected crystallographic orientation. A first pair of low resistivity zones are formed in one surface of the body and a second pair of low resistivity zones are formed in the opposite surface thereof. The low resistivity zones on one surface define the two opposed portions of a four portion closed figure, while the low resistivity zones on the opposite surface define the other two opposed portions thereof. Means are provided for making electrical connections to the respective end portions of the low resistivity zones on opposite surfaces of the body to form therefrom a complete bridge circuit, wherein the arms of each current path are disposed on opposite surfaces of the body and separated from each other by the high resistivity material thereof. The crystallographic orientation of the monocrystalline body and the conductivity-type of the low resistivity zones formed therein, are selected to assure that the longitudinal and transverse elastoresistance coefiicients of these zones are related so as to produce a maximum unbalance of the bridge circuit for the applied force. For example, the low resistivity zones may be selected to exhibit longitudinal and transverse elastoresistance coefficients which are either the same or different in polarity, magnitude or both depending upon the particular application of the device and the disposition of the low resistivity zones thereon.

As used throughout the specification and in the appended claims the term longitudinal with respect to the elastoresistance coefficient refers to the change in resistivity when current and strain are measured parallel to each other. Similarly, the term transverse with respect to the elastoresistance coeificient refers to the change in resistivity when current and strain. are measured perpendicular to each other.

The novel features believed characteristic of this invention are set forth with particularity in the appended claims. My invention itself, however, both as to its or ganization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in conjunction with the accompanying drawing in which:

FIGURE 1 is a diagrammatic illustration of a semiconductor strain sensitive device in accordance with one embodiment of this invention,

FIGURE 2 is a sectional view along the line 2-2 illustrating a portion of the low resistivity zone as a part of the strained member itself,

FIGURE 3 is a perspective view partly in phantom and partly in section illustrating the disposition of the low resistivity conducting zones on opposite surfaces of the strain sensitive device; and

FIGURE 4 is a diagrammatic illustration of a semiconductor strain sensitive device in accordance with another embodiment of this invention.

In FIGURE 1 a device 10 embodying the principles of this invention comprises a monocrystalline body 11 of high resistivity, wide band-gap semiconductive material. Body 11 may be composed of silicon, gallium arsenide, aluminum antimonide, or a similar wide band-gap material capable of achieving a wide resistivity range at room temperature. For example, the semiconductive material of body 11 should be capable of achieving a sufiiciently high resistivity at room temperature so that the ratio of the resistivity of the high resistivity material to the resistivity of a low resistivity zone established therein is at least 10 and preferably greater than 10 Thus, the bandgap of the semiconductive material must be sufiiciently wide that the concentration of intrinsic carriers is not too great to prevent achieving the required high resistivity at room temperature in the body 11, as well as allowing high impurity induced conductivity zones to be established therein.

The body 11 may be in the form of a circular disk, or rectangular plate, and will henceforth be described as high resistivity silicon in the form of a disk having two large-area plane parallel surfaces 12 and 13 respectively.

The body 11 is provided with a first pair of low resistivity zones 14, formed in the surface 12, and a second pair of low resistivity zones 15 formed in the opposite surf-ace 13. The zones 14 and 15 are very thin, preferably in the range of about -100 microns, and of a resistivity less than about 10 ohm centimeters and preferably in the range of about 0.1 to 0.001 ohm centimeter at room temperature. The pairs of low resistivity zones 14' and 15 are so related, one to the other, that the projection of all zones on to a single plane surface defines a four portion closed figure. The closed figure may have a wide variety of geometric configurations such as, for example, square, rectangular, circular, or the like, provided four portions may be defined thereby. For simplicity of explanation, the closed figure is illustrated in the drawing in the form of a square frame, wherein the pair of low resistivity zones 14 on the surface 12 define two opposed parallel sides, and the pair of low resistivity zones 15, perpendicular to the pair 14 on the opposite surface 13, define the other two opposed parallel sides. Thus, the orthogonally related portions of the closed figure are disposed on opposite surfaces of, and separated by, the high resistivity semiconductive material of the body 11. The four portions of the closed figure are designated by the reference numerals 16, 17, 18 and 19 respectively.

Means are provided for making substantially nonrectifying electrical connections between the respective end portions 20, 21, 22 and 23 of the low resistivity zones 14-, and 24, 25, 26 and 27, of the low resistivity zones 15 to form therefrom a complete bridge circuit. To this end, a conductor is passed through the body 11 to contact the ends of the oppositely disposed low resistivity zones. A suitable conductor 28 passes through the body 11 and electrically connects the end 20 of portion 18 to the end 24 of portion 16. Similar connections are provided to the ends 21-26, 2225 and 2327 of the opposite low resistivity portions 18-19, 17-16 and 17-19 respectively, to form a complete bridge circuit, as typified in the illustration of FIGURE 2. The conductor 28 may be connected to the ends of the respective low resistivity zones by means of a suitable solder 29 or other means known to the art.

Since as a practical matter it is extremely ditficult to obtain a completely nonrectifying connection to semiconductive material, the ends of each of the portions 16 to 19 are provided with an enlargement. These enlarged end portions provide for the recombination of some of the carriers which are injected due to any rectifying properties of the connections between the oppositely disposed ends of the low resistivity portions 16-19 or between the electrode connections thereto during operation, as well as providing for symmetrical current flow in the respec tive current paths of the bridge unit.

Two ends of the bridge circuit so established may be connected to a suitable voltage source and the other two ends connected to a suitable detecting means. For example, the ends 20 and 23 may be connected to the terminals 30 and 31 of a voltage source, shown schematically as battery 32. The remaining two ends 21 and 22 may be connected to a detecting means 33, such as a galvanometer, voltmeter, or the like. The bridge circuit so provided, therefore, has one resistance arm of each current path disposed on one surface of the monocrystalline body 11 and the other resistance arm thereof disposed on the opposite surface thereof; the two arms being separated by the high resistivity material of the body 11.

To achieve optimum sensitivity from the complete bridge-type strain sensitive device of this invention the resistance in the two portions of the bridge on the one surface should increase while the resistance of the two portions of the bridge on the opposite surface decreases fora given strain. When the device is used as a pressure diaphragm for measuring hydrostatic pressure for example, the low resistivity zones on one surface are loaded in tension and the low resistivity zones on the other surface are loaded in compression. Also in such a device, the predominant strain is such that the change in resistance due to the pressure is determined principally by the longitudinal elastoresistance coefficient of the low resistivity zones making up the bridge unit. The crystallographic orientation of the body 11 of such a device, therefore, and the conductivity type of the respective low resistivity zones are selected to assure that the longitudinal elastoresistance coefficient of these low resistivity zones is large and of the same polarity; the respective increase and decrease in resistance for a given strain being provided by the loading in tension of one pair of zones and the loading in compression of the opposite pair of zones. In addition it is also advantageous in such a pressure diaphragm device to provide that the transverse elastoresistance coefficient has the same polarity as the longitudinal elastoresistance coefficient or else be comparatively small in magnitude.

Alternatively, the pairs of low resistivity zones 14 and 15 may be provided on the same surface of the high re sistivity body 11 as shown particularly in FIGUREZ4, by suitable selection of crystallographic orientation of the body and conductivity-type of the low resistivity zones formed therein to provide two opposite portions of the closed figure with a large longitudinal elastoresistance coefficient of one polarity and the remaining two opposite portions thereof with a large longitudinal elastoresistance coefficient of opposite polarity. Under this condition, in a pressure measuring device, although all portions of the bridge unit are loaded in the same way, the respective increase and decrease in resistance of the various portions thereof is provided by the. opposite polarity longitudinal elastoresistance coefficients.

When the device is to be employed to measure uniaxial strain, optimum sensitivity is achieved by providing that the low resistivity zones exhibit longitudinal and transverse elastoresistance coefficients which are large and of opposite polarity. In this way the change in resistance in one arm of each current path is of opposite polarity to the change in resistance of the other arm of each path thereby providing the desirable increase and decrease in resistance of the respective bridge arms. For example, in such Operation the change in resistance of one pair of opposite portions of the closed figure is determined prinr cipally by the longitudinal elastoresistance coefficient of the low resistivity material while the change in resistance of the remaining pair of opposite portions of the figure is determined principally by the transverse elastoresistance coefficient. Although it is preferred, therefore, that the longitudinal and transverse elastoresistance coefficients be large and of opposite polarity for such. a uniaxial strain measuring device, it will be understoodthat an extremely sensitive device is provided as longas the longitudinal and transverse coefiicients are anisotropic, that is, as long as the coefficients differ from each other in either magnitude, polarity or both. Also, in such a uniaxial strain measuring device, the low resistivity zones forming the bridge-unit may be provided either on the same or opposite surfacesof a very high resistivity monocrystalline body of preselected crystallographic orientatron.

The piezoresistance coefficients are usually given for the axis of the crystal, however, it .has become.

well-known in the art that the piezoresistance coefficients may also be readily determined with respect to other crystal rotations. Thus, the piezoresistance coefficients, and hence the related elastoresistance coefiicients, may be determined for any given semiconductive material in known manner. Further details of the factors to be considered in making the determinations of the elastoresistance coefficients with respect to the different crystal orientations may be had by reference to vol. 6 of the book entitled Solid State Physics, Advances in Research and Application," pages -249, Macroscopic Sym:

metry and Properties of Crystals, published in 1958 by Academic Press, Inc., New York. Still further details about such coefficients may be 'had by reference to the article entitled The Piezoresistance Effect and Its Applications, by Lewis B. Hollander et al., published in March 1960, Review of Scientific Instruments, vol. 31, No. 3, pages 323-327.

An inspection of the piezoresistance coefiicients of various semiconductive materials shows that some materials have one large coefiicient while others have two large coefiicients, and that some coefiicients are positive while others are negative. For example, N-type conductivity silicon has a large negative longitudinal coefiicient' and also a large positive transverse coefiicient. Thus, depending upon the selected orientation of the body and the semiconductive material of which it is composed, the low resistivity zones 14 and 15 may be made either the same or different conductivity-type so as to achieve a large longitudinal elastoresistance coefficient for both pairs of zones, a large longitudinal elastoresistance coeflicient for one pair of zones and a large transverse elastoresistance coefficient for the other pair of zones, or a large longitudinal elastoresistance coeflicient for one pair of zones and small transverse coetlicient for the other pair. Depending both upon the application of the device and the disposition of the low resistivity zones therein, the crystallographic orientation of the body and the conductivity-type of the low resistivity zones formed therein are selected to provide longitudinal coefiicients which have the appropriate polarities and magnitude to produce maximum unbalance for an applied force. When these pairs of zones are on opposite sides of the body 11 with one pair loaded in tension while the other is loaded in compression, as in the pressure measuring device, there is freedom to choose an orientation and conductivity-type which gives the greater sensitivity and linear range of response for the device.

For example, in the drawing the high resistivity silicon body 11 is shown having an orientation along the 100 direction as indicated by the arrow A. Since N-type conductivity silicon exhibits large and opposite polarity longitudinal and transverse elastoresistance coefiicients, the two pairs of zones 14 and 15 may be suitably impregnated with an impurity material to render them of 'N-type conductivity and of a resistivity at room temperature less than about ohm centimeters. The low resistivity zones 14 and will then exhibit the desired large longitudinal elastoresistance coefficients of one polarity, and at the same time large transverse elastoresistance coefiicients of opposite polarity, so that the device may be conveniently employed as either a pressure or uniaxial strain measuring device.

To obtain balance for the bridge circuit, the ratio of the resistances of the two low resistivity portions, defining one current path, should be equal to the ratio of the resistances of the two low resistivity portions which define the other current path. For example, the following relationship should be satisfied:

where the terms R R R and R refer respectively to the resistance values of the four resistance portions 16-19 of the low resistivity closed figure forming the complete bridge circuit. This may be conveniently provided by making all four portions 1649 of substantially equal dimensions and resistivity, such as by establishing a uniform low resistivity closed figure having a square or circular configuration for example. It Will be understood, however, that the dimensions and resistivity of the respective portions 1649 may be different, and the above relationship satisfied, utilizing a wide variety of closed figure configurations.

It will be apparent from the above relationship and the foregoing detailed description that greatest unbalance of the bridge is achieved when the change in resistance in the portions 16 and 19 is large and opposite to the change in resistance in the portions 17 and 18. For example, a maximum detector output for a given stress results when the portions 16 and 19 show a large increase in resistance While simultaneously the portions 17 and 18 show a large decrease in resistance or vice versa.

This condition is fully satisfied in the silicon strain sensitive device shown in the drawing with an orientation of body 11 in the l00 direction, and low resistivity zones 14 and 15 rendered of N-type conductivity. For example, the portions 17 and 18 may be disposed parallel to the axis while the portions 16 and 19 may be disposed parallel with the 0l0 axis, as shown particularly in FIGURE 3. Thus, since the transverse elastoresistance coefiicient of N-type silicon in the 100 direction is large and positive, there is an increase in resistance in the portions 16 and 19 for a uniaxial strain in a direction parallel to the l00 direction. Similarly, the longitudinal coefficient of the N-type conductivity silicon in the 100 direction is large and negative, resulting in a decrease in resistance in the portions 17 and 18 for such a uniaxial strain. In like manner, for a pressure measuring device, the change in resistance is determined principally by the longitudinal elastoresistance coefiicient for both pairs of low resistivity zones 14 and 15 with the desired opposite polarity of this change being provided by the compression and tension loading of the zones respectively. The resulting unbalance of the bridge circuit as a result of either hydrostatic pressure or a uniaxial stress may be detected by the detecting means 33 in a well-known manner.

The improved semiconductor strain sensitive device 10 is fabricated in the following general manner in accordance with a specific embodiment of the method of this invention.

A high purity, wide band-gap monocrystalline body or Wafer, such as of silicon, gallium arsenide, or the like, is provided having a preselected crystallographic orientation. The size and thickness of the body are determined by the magnitude of the load to be measured by the device and the sensitivity desired. The body is converted to high resistivity by diffusing thereinto a deep-leve1 impurity such as copper or gold to provide a resistivity therefor of greater than about 10,000 ohm centimeters. Two opposite portions of a four portion closed figure are converted to low resistivity on one surface of the body, as by maskdiffusing a conductivity determining impurity thereinto. The other two opposed portions of the closed figure are converted to low resistivity on the opposite surface of the body in similar manner. The concentration of impurity should be sufficient to provide these portions With a resistivity less than about 10 ohm centimeters. Further, the conductivity determining impurity is selected to render the low resistivity portions on both surfaces of the body, for the preselected orientation, of a conductivity-type which exhibits a longitudinal elastoresistance coefiicient which is large and of one polarity. Preferably, the transverse coefficient is provided With the same polarity as the longitudinal coefiicient, for a pressure device, and with opposite polarity for a uniaxial force measuring device. For example, for a silicon body having an orientation in the 100 direction this latter condition is satisfied by indiifusing a donor impurity, such as phosphorous into selected surface adjacent portions thereby rendering these portions of N-type conductivity and low resistivity since both the longitudinal and transverse elastoresistance coefiicients of N-type silicon are large and are of opposite polarity.

The ends of the oppositely disposed low resistivity portions are electrically connected together to form a complete bridge circuit therefrom. Small holes are drilled through the ends of the low resistivity portions on one surface of the body, through the semiconductive material of the body itself, and through the ends of the low re sistivity portions of the opposite surface of the body. A metal is suitably evaporated over the ends of the re spective low resistivity portions on the opposite surfaces of the body and conductors, such as metal wires or the like, passed through the holes and suitably connected,

as by soldering, onto the metallized ends of the low resistivity portions. Required balancing of the bridge unit so formed is provided by removing some of the low resistivity material from one or more portions thereof such as by etching or the like, Preferably, such removal may be provided by an electrolytic etching treatment since such treatment is conveniently controllable.

The following specific examples are given by way of illustration only, as an aid in the practice of the method of this invention and are in no way intended as a limitation thereof.

Example I A monocrystalline disk of silicon having a diameter of about /1 inch and a thickness of about A; inch is cut from a silicon body having unremoved impurities in an amount less than about 10 atoms per cubic centimeter. The disk is cut from the parent crystal so as to have a crystallographic orientation in the l direction. A layer of gold of about 0.1 micron thick, is evaporated on both surfaces of the disk and the disk thereafter heated to a temperature of about 1200 C. for about 50 hours to cause the gold to be diffused thereinto to provide the semiconductive material thereof with a resistivity greater than about 10 ohm centimeters. For example, after heating at 1200 C. for 50 hours the resistivity of the disk is about 10 ohm centimeters.

The entire surface of the disk is then caused to be oxidized such as by heating in an oxidizing atmosphere for about two hours at 1100 C. Selected portions on opposite surfaces of the disk are then unmasked by etching away the oxide coating. The selected etched portions are in the shape of two parallel, spaced-apart strips, each inch long and A inch wide having enlarged ends on one surface, and two similarly shaped and dimensioned spaced-apart parallel strips on the opposite surface of the disk. The respective pairs of strips are orthogonally related to each other so as to form between them a generally square frame-like figure; each of the strips of the figure having a generally dumbbell configuration due to the enlarged end portions.

The disk is then heated in the presence of phosphorous to a temperature of about 1200 C. for four days, to diffuse the phosphorous into the surface adjacent regions of the unmasked portions, rendering them of N-type conductivity and with a resistivity of about 0.01 ohm centimeter =and a thickness of about 15 microns. Holes are then drilled through the enlarged ends of the low resistivity strips, a layer of gold is evaporated onto the enlarged ends of the strips, and thereafter ametal wire passed through the holes and soldered into the holes and onto the enlarged metallized ends of the strips to form a complete bridge-type strain measuring device.

Example 11 A monocrystalline disk of silicon, having a diameter of about /1 inch and a thickness of about Ms inch, is cut from a monocrystalline body of silicon having unremoved impurities in an amount no greater than atoms per cubic centimeter. The disk is cut from the silicon body so as to have a crystallographic orientation in the 100 direction.

The entire surface of the disk is then oxidized by heating in an oxygen or air atmosphere for about two hours at a temperature of about 1100 C. Selected portions of this oxide coating are then etched away. The selected portions are in the form of two spaced-apart parallel strips having enlarged end portions on one broad surface of the disk which lie parallel with the 100 direction of the crystal, and two similarly shaped, parallel, spacedapart strips on the opposite broad surface of the disk which lie parallel with the 010 direction of the crystal. The unmasked strips are thus orthogonally related to each other and so positioned as to form between them a square frame-like figure; each of the strips of the figure having a generally dumbbell configuration due to the enlarged end portions.

Four small holes about /64 inch in diameter are drilled through the disk and through the enlarged end portions of the strips. The disk is thereafter heated in the pres ence of phosphorous for about four days at a temperature of about 1200 C. to render the regions not protected by the oxidized coating of N-type conductivity and a resistivity of about 0.01 ohm centimeter to a thickness of about 15 microns.

The remaining oxide coating is then removed by etching and a layer of gold about 0.1 micron thick is evaporated over both sides of the disk. The disk is then heated for about 50 hours at a temperature of about 1200 C. to diffuse the gold into the disk. After heating for about 50 hours the resistivity of the disk with the exception of the low resistivity phosphorous diffused strips is about 10 ohm centimeters. The entire disk, therefore, with the exception of the narrow strips forming the low resistivity square frame figure, exhibits a very high resistivity.

Nickel is thereafter evaporated onto the enlarged ends of each of the low resistivity N-type conductivity strips, metal wires passed through the small holes, and the wires soldered into the holes and onto the enlarged and metallized ends of the strips to formtherefrom a complete bridge circuit. The bridge circuit so formed is balanced by electrolytically etching one of the arms.

There has been described hereinbefore a strain sensitive device of the semiconductor complete bridge-type, and a method of fabricating such a device, having improved characteristics. The piezoresistance elements which make up the bridge unit are formed within the strained member itself, thus avoiding the requirement of using cement or other bonding material. Since in one embodiment two arms of the bridge unit are loaded in tension while the other two are loaded in compression, there is great freedom to choose the orientation of the member which gives the greatest sensitivity and linear range of response. Further, the low resistivity zones which define the complete bridge unit may be made very thin so that they may be heavily impregnated with impurity, to achieve optimum piezoresistance effects and temperature stability, and yet yield a high impedance bridge unit.

While this invention has been described with respect to particular embodiment and specific examples thereof by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. The method of making a semiconductor strain sensitive device which comprises: providing a monocrystalline body of high purity wide band-gap semiconductive material having a preselected crystallographic orientation; converting selected portions only of a surface-adjacent region in the form of a four portion closed figure bridge circuit to low resistivity and of a conductivitytype with respect to the crystallographic orientation of the body to provide the said portions of said figure with longitudinal and transverse elastoresistance coefficients which are related as to polarity and magnitude toprovide maximum bridge unbalance for an applied force; and diffusing a deep-level impurity into the remaining portions of said body to impart thereto a very high resistivity to provide electrical isolation of said zones in said body.

2. The method of claim 1 wherein the conductivitytype of the respective low resistivity portions are so exlected that two opposite portions of the closed figure exhibit a large longitudinal elastoresistance coefficient of one polarity and the remaining two opposite portions of 9 the closed figure exhibit a large longitudinal elastoresistance coeflicient having opposite polarity.

3. The method of claim 1 wherein the conductivitytype of the low resistivity portions are selected to exhibit longitudinal and transverse elastoresistance coefiicients which are large and of opposite polarity.

The method of claim 1 wherein the Wide band-gap semiconductive material is silicon and the deep-level impurity diffused the'reinto to impart said high resistivity is gold.

5. The method of making a semiconductor strain sensitive device which comprises: providing a monocrystalline body of high purity Wide band-gap semiconductive material having a preselected crystallographic orientation; converting two selected portions of the surface-adjacent region of one side of said body to low resistivity and of conductivity-type with respect to the orientation of said body to provide such portions with large longitudinal and transverse elastoresistance coefiicients which are of opposite polarity; similarly converting two selected portions of the surface adjacent region of the opposite side of said body, the selected portions converted to low resistivity being so related that such portions define a four portion closed figure wherein two opposite portions are disposed on one surface of the body and the remaining two opposite portions are disposed on the opposite surface thereof; diffusing a deep-level impurity material into the remaining regions of said body to impart to said body very high resistivity such that the resistivity ratio between the high and low resistivity portions of said body is greater than the high resistivity of said remaining regions providing electrical isolation of said low resistivity portions; and electrically connecting the ends of the low resistivity portions on one surface of said body to the related ends of the low resistivity portions on the opposite surface thereof to form a complete unitary bridge circuit therefrom.

6. The method of making a semiconductor strain sensitive device which comprises: providing a high purity monocrystalline body of semiconductive material having a preselected crystallographic orientation; oxidizing the surface of said body; etching away two selected portions of said oxidized surface on two opposite surfaces of said body, said selected portions being so related that such portions define a four portion closed figure wherein two opposite portions thereof are disposed on one surface of said body and the remaining two opposite portions are disposed on the opposite surface thereof; drilling holes through said body and through the ends of the etched portions; diflusing a conductivity determining impurity material into the etched portions, said impurity being selected to impart a conductivity type thereto for the orientation of said body to provide said portions with low resistivity and large longitudinal and transverse elastoresistance coefiicients of opposite polarity; etching away the oxide coating from the remaining surfaces of said body; difiusing a deep-level impurity into the remaining regions of said body to impart thereto a resistivity greater than 10 ohm centimeters .at room temperature, the high resistivity of said remaining regions providing electrical isolation of said portions of loW resistivity; and electrically connecting the oppositely disposed end portions of the closed figure through said holes to form from said low resistivity portions a complete unitary 'bridge circuit wherein one resistance arm of each current path is disposed in one surface of said body and the other resistance arm of each current path is disposed in the opposite surface thereof.

References Cited UNITED STATES PATENTS 3,270,554 9/1966 Pfann 338-2 X WILLIAM I. BROOKS, Primary Examiner. 

1. THE METHOD OF MAKING A SEMINCONDUCTOR STRAIN SENSITIVE DEVICE WHICH COMPRISES: PROVIDING A MONOCRYSTALLINE BODY OF HIGH PURITY WIDE BAND-GAP SEMICONDUCTIVE MATERIAL HAVING A PRESELECTED CRYSTALLOGRAPHIC ORIENTATION; CONVERTING SELECTED PORTIONS ONLY OF A SURFACE-ADJACENT REGION IN THE FORM OF A FOUR PORTION CLOSED FIGURE BRIDGE CIRCUIT TO LOW RESISTIVITY AND OF A CONDUCTIVITYTYPE WITH RESPECT TO THE CRYSTALLOGRAPHIC ORIENTATION OF THE BODY TO PROVIDE THE SAID PORTIONS OF SAID FIGURE WITH 